III-N single crystals are of great technical importance. A multitude of semiconductor devices and optoelectronic devices such as power components, high-frequency components, light-emitting diodes and lasers are based on these materials. Epitaxial crystal growth on a starting substrate is frequently carried out when producing such devices, or a template is initially formed on a starting substrate, onto which III-N layers or respectively III-N boules can be subsequently grown by further epitaxial growth. III-N substrates or in particular foreign substrates can be used as starting substrates. When using foreign substrates, stresses and cracks within a III-N layer can occur during the growth due to the differences between the thermal expansion coefficients of starting substrate and epitaxial layer. Thicker layers of up to 1 mm can also be grown with the aid of partially structured interlayers composed of WSiN, TiN or SiO2, wherein said thicker layers can be subsequently separated as free-standing layers which typically have plastic, concavely bent c lattice planes and surfaces. In particular in case it is dispensed with an intermediate layer in such a process, at or above the interface between starting substrate and epitaxial III-N layer vertical and horizontal micro-cracks form, which can expand over time and which can lead to breaking of the GaN layer during or after the cooling process.
From investigations by Hearne et al., Applied Physics Letters 74, 356-358 (1999) it is known that during the deposition of GaN on a sapphire substrate an intrinsic tensile stress builds up which increases with the growth. An in situ stress monitoring showed that the tensile stress produced by the growth cannot be measurably relaxed by annealing or thermal cycling. This means inter alia that a stress obtained at the end of the growth of the GaN layer will have the same value again after cooling and reheating to the same (growth) temperature. In Hearne et al. also an explanation of the background, relationships and possibilities for observation of extrinsic (namely generated by different thermal expansion coefficients between sapphire substrate and GaN layer) and intrinsic (namely generated by growth) stress is given.
In this regard Napierala et al. in Journal of Crystal Growth 289, 445-449 (2006) describe a process for producing GaN/sapphire templates onto which crack-free thin GaN layers are grown by being able to control the intrinsic stress in the gallium nitride through the setting of the density of gallium nitride crystallites in such a way that stresses in the thin layers can be released by bending. In this process, however, thick layers cannot compensate the pressure during the growth and tend to breaking despite the bending. Richter et al. (E. Richter, U. Zeimer, S. Hagedorn, M. Wagner, F. Brunner, M. Weyers, G. Trankle, Journal of Crystal Growth 312, [2010] 2537) describe a process for producing GaN crystals via Hydride Vapor Phase Epitaxy (HVPE) in which GaN layers having a thickness of 2.6 mm can be grown in a crack-free manner by setting the partial pressure of gallium chloride, wherein the obtained GaN layers exhibit a multitude of V-pits on the surface. A crystal grown with this process has a thickness of 5.8 mm, it however exhibits longer cracks. Brunner et al. in Journal of Crystal Growth 298, 202-206 (2007) show the influence of the layer thickness on the curvature of the epitaxial III-N layer. The growth of GaN and AlGaN, optionally with InGaN compliance layer, on GaN-sapphire template is investigated. It was found that for GaN and AlGaN with 2.8% and 7.6% of Al mole fraction the concave curvature increases during the growth, which according to observation accompanies the generation of a tensile stress (cf. FIG. 3). Furthermore, the concave curvature increases with rising aluminium content, accordingly the tensile stress further increases. In addition, the influence of a Si-doped indium gallium nitride layer on the growth of an AlGaN layer with 7.6% of Al mole fraction on a GaN buffer layer is shown. For this purpose on the one hand an AlGaN layer with 7.6% of Al mole fraction is directly grown onto a GaN buffer layer, and on the other hand a Si-doped indium gallium nitride layer as interlayer is grown onto a GaN buffer layer, wherein subsequently an AlGaN layer with 7.6% of Al mole fraction is grown onto the interlayer. It was thus shown that the deposition of a Si-doped indium gallium nitride layer onto a GaN buffer layer leads to compressive stress in the crystal. During this process the initially concave curvature of the GaN buffer layer is transformed into a slightly convex curvature in the course of a temperature reduction, and this convex curvature increases during the further growth by epitaxially growing an In0.06GaN layer within the same process. During the subsequent deposition of an Al0.076GaN layer onto this In0.06GaN layer a concave curvature is eventually obtained, which is comparatively lower than the resulting curvature without In0.06GaN interlayer.
E. Richter, M. Gründer, B. Schineller, F. Brunner, U. Zeimer, C. Netzel, M. Weyers and G. Trankle (Phys. Status Solidi C 8, No. 5 (2011) 1450) describe a process for producing GaN crystals via HVPE, wherein a thickness of up to 6.3 mm can be reached. These crystals exhibit slanted sidewalls and V-pits on the surface. Moreover, the crystal lattice has a concave curvature of approximately 5.4 m and a dislocation density of 6×10−5 cm2.
US 2009/0092815 A1 describes the production of aluminium nitride crystals having a thickness between 1 and 2 mm as well as aluminium nitride layers having a thickness of 5 mm. These layers are described as crack-free and can be used to cut colourless and optically transparent wafers having a usable area of more than 90% for the application in the production of devices and components.
The processes in the above-described prior art have in common that after growth and cooling-down III-N crystals are obtained which are subjected to strong extrinsic and intrinsic stress, whereby cracks or other material defects can develop, which limit the material quality and the processability towards III-N substrates.